The invention relates to instrumentation systems which employ scanning of a specimen, for example: scanning electron microscopes, thyroid scanning systems, fuel rod scanning systems, electron microprobe systems, and related scanning systems. In this particular embodiment, the invention relates to a scanning electron microscope system which is used in conjunction with an X-ray energy spectrometer (FIG. 1).
In X-ray energy spectrometers, a cathode ray tube is mormally used to display a continuous spectrum of X-ray energy. As illustrated in FIG. 2, the horizontal (x) axis of this display represents X-ray energy (calibrated in kilo-electron volts, keV) and the vertical (y) axis represents the number of detected X-ray events (calibrated in X-ray counts). The spectral peaks in this display represent the gaussiam distribution for characteristic X-ray energy events (lines) of the element or elements present in the specimen being analyzed. Large peaks in the display indicated that there is a relatively large concentration of a particular element in the sample, while small peaks indicate that only traces of an element are present.
A common problem encountered when working with small concentrations of an element is that the ratio of peak counts to background continuum (noise) counts is small. This condition appears as a decrease in the signal-to-noise ratio (i.e., a loss of image contrast and sharpness), when employing a scanning electron microscope (SEM) to generate an x-ray map of the trace elements in a specimen (see FIG. 2). One solution to this problem has been to subtract the majority of background counts from the peak counts and thereby improve the resolution of the SEM x-ray image.
Substraction of background counts has previously been achieved by using a single channel analyzer (SCA) and a digital "count rate discriminator" circuit. The SCA was used to monitor X-ray events occurring in the energy band encompassing the elemental peak of interest, such as band b.sub.1 to b.sub.2 in FIG. 2. The output of the peak SCA was applied to a pulse rate discriminator circuit consisting of a one-shot and a comparator gate or flip-flop. The one-shot pulse width was selected by the operator to simulate a "mean background" count rate. When the element peak of interest was scanned by the SEM, the peak count rate would exceed the mean background rate and the comparator would enable output pulses from the discriminator circuit. The count rate discriminator therefore worked by inhibiting output pulses to the SEM video monitor whenever the observed x-ray count rate within the energy window of interest dropped below an operator-selected level, and enabling output pulses to the SEM monitor whenever the observed count rate exceeded that level.
Such a background subtraction scheme embodies a number of undesirable features, such as relying on a mean or average background approximation and depending on the observed background count rate to remain constant as the specimen is scanned. Considerable operator judgement may be required to optimize system performance under changing operating conditions. In addition, the efficiency of the image enhancement provided by this method decreases under conditions of low overall count rates and/or poor peak-to-background ratios and is also unsuitable for use at video scan rates. Finally, the output pulse rate from count rate discriminators is non-linearly related to the net peak count rate.